Servo systems are used in disc drives for controlling the arm assembly to position a head on the arm assembly over a selected track of a disc. A "normal" tracking system is used to accurately maintain the head over the selected track.
When disc drives must change from one track to another selected track on the disc, it is important to accurately reposition the head over the newly selected track as quickly as possible to minimize the access time. The normal tracking system is not suitable for this purpose.
Most disc drives use a separate "coarse" track seeking system that takes over servo control from the normal tracking system to rapidly move the head into approximate alignment with the selected track. Once the head is so approximately positioned, the normal tracking system takes over servo control from the coarse track seeking system to accurately align the head precisely over the selected track.
Once the normal tracking system accurately positions the head over the newly selected head for a duration that is sufficient to prevent mistracking, the normal tracking system allows the drive to perform the read or write operation that it has been requested to do.
The normal tracking system has a "fine" track seeking operation that uses the proximity of the head to the centerline of the selected track to determine when the head has sufficiently settled on the selected track. The normal tracking system then declares the head to be on track when the head has remained within a selected proximity of the track centerline for a selected entrance period.
The entrance period is selected to insure that the head will remain accurately positioned on the selected track before the drive is allowed to read or write. For example, the entrance period may correspond to the period of a selected number of sectors on the selected track.
The criteria used by the normal tracking system to determine when the head is on track is referred to as the "fine" track entrance criteria. Normally, the fine track entrance criteria is set for a particular type of disc drive by empirically measuring a criteria for a sample of drives that is conservative enough to insure that any off-track or read errors cannot occur with any of the tested drives.
Because the selected fine track entrance criteria of the type described above must be conservative enough to protect the average disc drive against off-track and read errors under all operating conditions, optimal fine track seeking is not possible under normal conditions. That is, the duration of the number of track sectors selected for the entrance period is greater than necessary for normal operation. This causes a longer than necessary track acquisition period.
Among the factors that cause the selected criteria to be excessive for normal operation is the variation of stability from unit to unit of the heads used in the disc drives. Also, some arm assemblies are more susceptible to tracking problems during settling than others.
Furthermore, the environment in which the disc drives are used can seriously affect their track seeking characteristics. For instance, if the ambient temperature rises, the damping and stiffness of the shock mounts holding the head/disc assembly of the disc drive can change appreciably. This change in damping and stiffness adversely affects the track seeking characteristics of the tracking servo system during the settling portion of the seek operation.
One approach to adapting the disc drive entrance criteria to changes in ambient temperature is to add a selected time to the entrance period when a certain temperature is sensed. This approach helps prevent offtrack and read errors caused by temperature rise, but the resulting fine track seeking operations are still performed at a rate that is far from optimal.